Ceramic coated metal substrates for electronic applications

ABSTRACT

The present invention is directed to a ceramic coated metal substrate having improved processibility characteristics in the manufacture of electronic devices, as for example electronic circuits, and to processes for manufacture of such devices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ceramic coated metal substrates for use inelectronic applications. More particularly, this invention relates tosuch coated substrates having improved processibility characteristics inthe manufacture of electronic devices, as for example electroniccircuits.

2. Prior Art

In the manufacture of electronic devices, it is standard to mount orform the various electronic components which comprise the circuits ofthe devices on a substrate. Various materials have been suggested foruse as the substrate. For example, relatively large circuits such asthose employed in radios, televisions, computers and the like aregenerally produced on organic substrates, as for example, reinforcedthermosetting resins, reinforced phenolic resins and reinforced epoxyresin laminates.

The organic plastic circuit boards have some advantages. For example,such boards are relatively inexpensive, can be manufactured in almostany desired size with flat smooth surfaces and have reasonably goodphysical strength. Such organic plastic circuit boards also display anumber of deficiencies which greatly limit their utility. For example,these materials are heat sensitive and cannot be exposed to hightemperatures, i.e. temperatures in excess of 400° C. Thus, metalcircuitry and the like must be formed on the surface of the board usinglow temperature deposition. Moreover, organic plastic substrates cannotbe used with newer methods of applying circuits to non-conductivesubstrates which are lower in cost and provide improved reliability andelectrical accuracy but which demand firing temperatures in excess of600° C. Another disadvantage of plastic circuit boards is thatresistors, capacitors, and the like must be manufactured as discretecomponents in separate manufacturing operations, and then individuallymounted on the board. In addition to the wide variety of manufacturingprocesses required to make a low temperature circuit board, the use ofdiscrete components severely limits the packing density which can beachieved.

The disadvantages of plastic circuit boards have led to the developmentof circuit manufacturing techniques which allow direct formation ofcomponents (conductors, resistors, capacitors, and the like) on thesurface of the substrate, (see M. L. Topfer, "Thick-FilmMicroelcctronics", Van Nostrand Reinhold, N.Y., 1971, and F. N.Sinnadurai, "Handbook of Microelectronics Packaging and InterconnectionTechnologies", Electrochemical Publications Limited, Great Britain,1985). The materials used to form these so called process inducedcomponents (PIC) are usually prepared in the form of inks comprisingmetal or ceramic powders, glass powders, polymer binders, and a liquidcarrier which are printed on the substrate and heated to drive off theorganic binder and fuse the remaining components to the substrate. Thefiring temperatures required for this process typically range from 600°C. to 950° C. considerably in excess of the degradation temperatures ofany organic plastic circuit board. The components prepared at these hightemperatures generally exhibit very good reliability and electricalaccuracy in addition to being relatively low in cost.

Other circuit board substrates have been fabricated out of ceramicsubstrates. A variety of ceramic materials are commercially availablefor use as high temperature substrates. Most notable is aluminum oxide.Examples of such substrates are described in Kuzel, R. et al.,"Ceramic-Coated Copper Substrates for Hybrid Circuits", Hybrid CircuitsNo. 4, pp. 4-9 (1984). These substrates composed of aluminum oxide havethe advantage that they have excellent high temperature resistance andmay be fired and refired at high temperatures, for example, temperaturesof from 600° C. to 900° C. Substrates formed from aluminum oxide arerelatively inexpensive when the wafers are small and of simple shape,and has therefore become something of an industry standard. However,these materials also suffer from several disadvantages. An inherentdisadvantage of ceramic substrates is that they are quite fragile. Thisplaces a significant size limitation on their use and, oftentimes,special fixturing is required to prevent damage while in use. Anotherdisadvantage of ceramic substrates is that they can be quite difficultand costly to machine. These aspects of ceramics generally limit theiruse to small, rectangular, single sided circuits which have very highreliability and precision.

Still other circuit boards have been fabricated out of porcelein coatedmetals such as steel and copper clad Invar. Illustrative of suchmaterials are those described in U.S. Pat. No. 4,256,796; Onysheevych,L.S., et al., "Manufacturing Steps in the Production of Porcelain-EnamelPC Boards", RCA Review, Vol. 42, pp. 145-158 (1981); Nobuo, I., et al.,"Thick Film Circuit on Bent Porcelain-Bent Substrate", Int. J. HybridMicroelectron., 5(2), pp. 1 to 8 (1982); Hugh, S. C., "MultilayerThick-Film Circuits on Porcelainized Steel Substrates", Int. J. HybridMicroelectron., 4(2), pp. 326-330 (1981); Hang K. W., et al., "LowExpansion Procelain-Coated Copper-Clad Invar Substrates", RCA Review,Vol. 45, pp 33-48 (1984); Prabhu, A. N., et al., "Optimization of RCAPorcelain for Compatibility With Thick Films", RCA Review, Vol. 42, pp.221 to 237 (1981); Hughes, E. W., "Status Report on Porcelain EnameledMetal Substrates", Ceramic Eng. Soc. Proc. 5, p. 219-220 (1984);Schabacker, R. B., "The Multiplicity of Variations in PEMS", Appliance,39(6), pp. 76-79 (1982); Schelhorn, R. L., "Metal Core Materials forThick Film Substrate Applications", Int. J. Hybrid Microelectron., 4(2),pp. 347-352 (1981); Hang, K. W., et al., "High-TemperaturePorcelain-Coated-Steel Electronic Substrates-Composition andProperties", RCA Review, Vol. 42, pp. 159-177 (1981); and Onyshkevych,L. S., "Porcelain-Enameled Steel Substrates for ElectronicApplications", Appliance, 38 (4), pp. 46-49 (1981). Although theseboards are not subject to thermal degradation as the organic plasticboards, and are substantially stronger than ceramic, they do suffer fromcertain process and use deficiencies. For example, when low temperatureporcelains are fired to the metal they melt and flow in such a way thata meniscus is formed at all edges. It therefore becomes very difficultto print circuitry over the resulting uneven surface. A further problemwith low temperature porcelains is that they soften and reflow at about600° C. which can lead to distortion of the overlying circuitry. Anotherproblem with porcelains is poor adhesion to the metal substrate duringuse because of substantial differences between the coefficients ofthermal expansion of the porcelain and the metal substrate.

Several attempts have been made to obviate the deficiencies of theporcelain-coated substrates. For example, U.S. Pat. No. 4,256,796discloses the fabrication of porcelain-coated metal circuit boardswherein the porcelain is a devitrified glass, and U.S. Pat. Nos.4,358,541 and 4,385,127 describe essentially alkali metal freeglass-ceramic coatings for use in the manufacture of circuit boards.While relatively effective these boards also suffer from severaldisadvantages. For example, the thermal coefficients of expansion (TCE)of the glass or devitrified glass when matched to the TCE of the metalcore results in a substrate with TCE's which are not a good match forTCE's of surface mounted components. For example, in the case ofpartially devitrified glass, the resultant fired coatings havedeformation temperatures greater than 700° C. and thermal coefficientsof expansion values greater than 11 ppm/°C. Although the high TCE isconsidered an advantage for adhesion to the metal core, it is adisadvantage when considering a good match to the surface mountedcomponents (typically alumina or silicon) which have TCEs in the 6 to 8ppm/°C. range.

Ceramics containing aluminum oxide, silicon oxide and magnesium oxideare known. For example, cordierite-based glass ceramic materials aredescribed in Mussler, B. H. and Shafer, M. W., "Preparation andProperties of Mullite-Cordierite Composites.", Ceramic Bulletin, Vol.63, No. 5, pp. 705-714-(1984), and references cited therein. As noted inCeramic Bulletin, when compared to alumina, presently the most commonlyused material, cordierite (2MgO-2Al₂ O₃.5SiO₂) offers lower dielectricconstant and thermal expansion, but has disadvantage of inferiormechanical properties. Because of the low thermal expansion and inferiormechanical properties of cordierite, it would be expected that thefabrication of electronic devices composed of metal substrates whichhave relatively high thermal coefficients of expansion coated withcordierite or cordierite based glass ceramic which have relatively lowthermal coefficients of expansion would not be feasible because of thelarge differences in thermal coefficients of expansion.

SUMMARY OF THE INVENTION

This invention relates to a ceramic coated article, having a metal coreand having on at least a portion of the surface of the metal core acoating of a glass ceramic, based on its oxide content and on the totalweight of the coating, comprising:

(a) from about 8 to about 26% by weight of magnesium oxide (MgO);

(b) from about 10 to about 49% by weight of aluminum oxide (Al₂ O₃) and;

(c) from about 42 to about 68% by weight of silicon oxide (SiO₂).

Several advantages flow from the invention. For example, theceramic/glasses used in this invention have a high temperature refiringcapability (>850° C.), and are air firable. Moreover, the article ofthis invention exhibits a composite thermal coefficient of expansionwhich is optimum for use in electronic devices, and exhibits a lowdielective constant which allows for use with high frequency circuitsand allows for greater applicability in electronic applications.Furthermore, the ceramic/glasses used in this invention exhibit strongadhesion to the metal substrate after firing and are very resistant tothermal stress. This avoids breakdown of the devices formed from thearticle of this invention when such articles are exposed to hightemperatures normally encountered in the operation of electronicdevices. This resistance to thermal stress is indeed surprising in viewof the relatively large difference in the thermal coefficient ofepansion of the metal substrate and the ceramic glass, and the priorteachings that the metal and coating coefficients of expansion must bematched to produce good adhesion.

Lastly, the combination of relatively low TCE coating (TCE usually lessthan about 4 ppm/°C.) with the relatively high TCE metal (TCE usually 10ppm/°C.) produces a composite coated substrate having a TCE within thepreferred range (usually from about 6 to about 8 ppm/°C.) which allowsfor a good TCE match between the substrate and surface mounted componentwhich are usually alumina or silicon having TCE's in the range of from 6to 8 ppm/°C.

Yet another aspect of this invention comprises a process formanufacturing a glass/ceramic coated article having a metal core andhaving on at least a portion of the surface of the metal core a coatingof a glass ceramic, said process comprising:

(a) heating a metal substrate in the presence of oxygen at a firsttemperature for a time sufficient to form any amount of an oxide layeron the surfaces of said substrate;

(b) applying to all or a portion of the surfaces of said substrate asuspension comprising one or more organic solvents, one or more heatdegradable polymeric binders and a calcined mixture of finely dividednon-conductive materials comprising:

(i) from about 8 to about 26% by weight of MgO;

(ii) from about 10 to about 49% by weight of Al₂ O₃ ; and

(iii) from about 42 to about 68% by weight of SiO₂ ;

(c) heating the coated/metal substrate combination of step (b) at asecond temperature for a time sufficient to remove substantially all ofsaid solvents from the applied suspension;

(d) heating said coated/metal substrate combination of step (c) at athird temperature for a time sufficient to degrade substantially all ofsaid binders in said applied suspension;

(c) heating the coated/metal substrate combination of step (d) at afourth temperature for a time sufficient to sinter said non-conductivematerial to form a device comprising a metal substrate having apredetermined pattern of glass/ceramic material bonded to one or moresurfaces thereof, said material comprising (on an oxide basis)

(i) from about 8 to about 26% by weight of MgO;

(ii) from about 10 to about 49% by weight of Al₂ O₃ ; and

(iii) from about 42 to about 68% by weight of SiO₂.

(f) heat treating said device at a fifth temperature for a timesufficient to recrystallize any residual glass contained in saidmaterial to any extent.

The process of this invention provides for greater selectivity in theapplication of the glass/ceramic materials to specific sites on asubstrate which provides for greater freedom in the manufacture ofdevices. After processing in accordance with this invention the coatingcontains crystallized glass/ceramic which is strongly adherent to themetal core and is suitable as a substrate for processed inducedcomponents.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the drawings

FIG. 1 is a perspective view of an embodiment of the ceramic coatedsubstrate of this invention.

FIG. 2 is an enlarged latitudinal sectional view of the substrate ofFIG. 1 taken along line 2--2.

FIG. 3 is a perspective view of an embodiment of the ceramic coatedsubstrate of this invention having an electrical circuit on the surfacethereof.

FIG. 4 is an enlarged latitudinal sectional view of the substrate ofFIG. 3 taken along line 4--4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, a coated metal substrate in accordance withthe present invention is indicated at 10. Substrate 10 includes a metallayer 12 having a number of through holes 14 for surface mounting otherelectronic components and for receipt of interconnecting devices forproviding electrical connection between device 20 formed from substrate10 and other electronic devices; substrate 10 also includes holes 28 forconnecting the substrate to supporting structure (not depicted), and anon-conductive layer 16 composed of the ceramic glass of this inventionheat bonded to selective portions of the top and/or bottom surfaces ofmetal substrate 12. In the preferred embodiments of the inventiondepicted to FIGS. 1 and 2, metal layer 12 consists of an elongated solidmetal strip. Metal layer 12 is composed of a metal such as copper,nickel, cobalt, iron, aluminum and the like, metals, and alloys thereofwhich is resistant to oxidation when heated at high temperature in thepresence of oxygen. Preferred for use in the practice of this inventionare metals which can be exposed to temperatures in excess of about 1100°C. in an oxidizing atmosphere without excessive spalling and which havethermal coefficients of expansion ≧10 ppm/°C., preferably ≧12 ppm/°C.over the temperature range of from about 50° C. to about 250° C.Illustrative of such preferred alloys are those alloys based on cobalt,nickel, and iron and which optionally include aluminum and whichpreferably also include chromium. Particularly preferred metals for usein the construction of metal layer 12 are nickel based alloys as forexample, the nickel alloy HAYNES® alloy No. 214 available from HaynesCorporation and the iron based alloys, as for example, the iron basedalloys Alfa-IV™ and Fecralloy available from Allegheny Ludlum SteelCorporation.

In general, the thickness of layer 12 will vary from about 0.025 mm toabout 0.75 mm. In the preferred embodiments of the invention, layer 12is from about 0,075 mm to about 0.6 mm in thickness and in theparticularly preferred embodiments is from about 0.125 mm to about 0.5mm in thickness. Amongst these particularly preferred embodiments, mostpreferred are those embodiments in which the thickness of layer 12 isfrom about 0.2 mm to about 0.4 mm.

Non-conductive layer 16 is bonded selectively to the bottom and topsurfaces of metal substrate 12 to provide the desired effect. In thisinvention, layer 16 is composed of a ceramic/glass composition. Ingeneral, ceramic/glass coating 16 will include on an oxide basis basedon the total weight of the coating of (a) from about 8 to about 26% byweight of MgO; (b) from about 10 to about 49% by weight of Al₂ O₃ ; and(c) from about 42 to about 68% by weight of SiO₂. In the preferredembodiments of the invention, useful ceramic-glass coatings will includeon an oxide basis (a) from about 9 to about 22% by weight of MgO; (b)from about 16 to about 45% by weight of Al₂ O₃ ; and (c) from about 43to about 63% by weight of SiO₂. In the particularly preferredembodiments of the invention, useful ceramic-glass coating will includeon an oxide basis (a) from about 10 to about 18% by weight of MgO; (b)from about 23 to about 40% by weight of Al₂ O₃ ; and (c) from about 44to about 58% by weight of SiO₂. Amongst these particularly preferredembodiments of the invention, most preferred are those embodiments inwhich useful ceramic-glass coating includes (a) from about 12 to about14% by weight of MgO; (b) from about 30 to about 35% by weight of Al₂ O₃; and (c) from about 45 to about 52% by weight of SiO₂.

The glass/ceramic for use in this invention may be any one of theglass/ceramics based on ternary metal oxide systems containing varyingamounts of the three components, MgO, SiO₂ and Al₂ O₃. Illustrative ofuseful ternany metal oxide systems are cordierite (2MgO-2Al₂ O₃ -5SiO₂),sapphirine (4MgO-5Al₂ O₃ -2SiO₂) and the like. In the preferredembodiments of the invention, the glass/ceramic coating will containvarying amounts of cordierite (2MgO-Al₂ O₃ -5SiO₂) but in addition mayinclude sapphirine (4 MgO-5Al₂ O₃ -2SiO₂).

In the preferred embodimens of this the ceramic coating also includes(on an oxide basis) boron oxide and one more alkali metal or alkalineearth metal oxides, preferably an alkali earth metal oxide such as Li₂O, Na₂ O and K₂ O and more preferably Li₂ O. These preferred oxidecomponents are derived from "fluxing and sintering agents" added to theglass/ceramic during its manufacture. In the preferred embodiments ofthe invention, the amount of alkali metal or alkaline earth metal oxidesis from about 1 to about 4 weight percent based on the total weight ofAl₂ O₃, MgO and SiO₂ in the composition and the amount of boron oxide isfrom about 3 to about 13 weight percent on the aforementioned basis. Inthe particularly preferred embodiments of the invention, the amount ofthe alkali metal or alkaline earth metal oxides is from about 1 to about3 weight percent, and the amount of boron oxide is from about 4 to about10 weight percent. Amongst these particularly preferred embodiments,most preferred are those embodiments in which the amount of alkali metalor alkaline earth metal oxide is from about 2 to about 3 weight percent,and the amount of boron oxide is from about 5 to about 7 weight percent.

The glass/ceramic coating may also optionally include from 0 to 11% byweight based on the total weight of Al₂ O₃, MgO and SiO₂ in thecomposition of one or more oxides which function as nucleating agents.Illustrative of such agents are transition metal and transition earthmetal oxides, such as TiO₂, ZrO₂, Y₂ O₃, Ta₂ O₅, La₂ O₃ and the like.Preferred nucleating agents are TiO₂ and ZrO₂, and the most preferrednucleating agent is TiO₂. In the preferred embodiments of the invention,the amount of the nucleating agent is from about 2 to about 8 weightpercent, based on the total weight of MgO, Al₂ O₃ and SiO₂ in thecomposition, and in the particularly preferred embodiments of theinvention, the amount of the nucleating agents is from about 4 to about6 weight percent on the aforementioned basis.

An especially preferred coating contains cordierite (2MgO-2Al₂ O₃-5SiO₂) and spodumene (Li₂ O -Al₂ O₃ -4SiO₂), and optionally TiO₂ and/orsapphirine (4MgO-5Al₂ O₃ -2SiO₂). These coatings are characterized ashaving thermal coefficients of expansion of from about 1.9 to about 2.5ppm/°C. over a temperature range of from about 50° C. to about 250° C.

The thickness of non-conductive layer 16 can vary widely. In general,layer 16 is from about 0.020 mm to about 0.100 mm in thickness. In thepreferred embodiments of this invention, the thickness of layer 16 isfrom about 0.025 mm to about 0.085 mm, and in the particularly preferredembodiments is from about 0.030 mm to about 0.070 mm. Amongst theseparticularly preferred embodiments of the invention, the thickness oflayer 16 is from about 0.040 mm to about 0.060 mm.

As depicted in electronic device of 20 FIG. 3 and FIG. 4, a conductivecircuit pattern 18 which is optionally applied selectively to the topand bottom of surfaces of layer 16 in such a manner to obtain thedesired electronic effect. In the preferred embodiments of thisinvention as depicted in these Figures, conductive layer 18 consists offinely divided metal which has been sintered and heat bonded to thesurface of non-conductive layer 16, in a desired circuit pattern. Thetype of metal which can be used in the construction of pattern 18 canvary widely, and can be any type of metal normally used in the formationof electrical circuits. Illustrative of useful metals are copper,nickel, palladium, platinum, silver, aluminum, gold and the like, andalloys thereof.

Thickness of pattern 18 can vary widely. Usually layer 18 has athickness of from about 0.005 to about 0.075 mm. In the preferredembodiments of the invention, the thickness of layer 18 is from about0.01 to about 0.06 mm, and in the particularly preferred embodiments isfrom about 0.015 to about 0.05 mm. Amongst these particularly preferredembodiments, most preferred are those embodiments in which the thicknessof layer 18 is from about 0.02 to about 0.03 mm.

Various other components are included in the electronic device 20depicted in FIG. 4. For example, the device includes a resistor 22bonded to the surface of non-conductive layer 16 between two conductivepaths 18. Resistor 22 can be composed of any material commonly used toform resistors, or for example, ruthenium oxide. The electronic deviceof FIG. 4 also includes a capacity element 24, which is a sandwichstructure comprising overlapping conductive patterns 18(a) and 18(b)bonded to and sandwhiching a layer 26 which is composed of finelydivided dielectric material such as one or more ferroelectric materials,for example, barium titanate, lead magnesium niobate strontium titanate,lead titanate, calcium titanate, calcium stannnate, lead magnesiumtungstate , barium potassium titanium niobate, calcium zirconte andsodium tantslate, either alone or in combination with one or moreglasses such as silicate, borate and germanate glasses capacity element24 can be formed by heat sintering and bonding finely divided materialsvery substantially the same techniques used to form layer 16 andconductive pattern 18 discussed below.

The ceramic coated substrate and the electronic circuit of thisinvention can be manufactured through use of the process of thisinvention. In the first step of the process of this invention, ifrequired, metal substrate 12 of a desired configuration is treated toremove burrs, sharp edges, and the like to facilitate later coating withthe ceramic. In the preferred embodiments of the invention, the metalsubstrate 12 is cleaned and degreased to remove foreign materials fromthe surface of substrate 12. Often metal substrates contain surfaceinhomogeneties which manifest themselves as pits in subsequent processsteps. In the preferred embodiments of the invention, the metal surfaceis treated to remove such surface inhomogeneties. Amongst the severaltechniques available for accomplishing this task, preferred arepolishing for example with sand paper, chemical etching and sandblasting. Metal substrate 12 is then heated in tee presence of anoxidizing atmosphere, preferably air, at a first temperature for a timesufficient to form any amount of a metal oxide layer on one or moresurfaces of said layer. The heat treatment of the metal is critical forthe formation of an adherent oxide layer which forms a bonding interfacefor the deposited ceramic coating. The oxide layer so form is preferablysubstantially homogeneous in order to prevent localized pitting orspalling. The temperature employed and the duration of the heating stepwill vary widely depending on the type of metal. In the preferredembodiments of the invention, metal substrate 12 is heated at atemperature of from about 800° C. to about 1250° C. for a period of from1/2 to about 24 hours, and in the particularly preferred embodiments,the metal substrate is heated at a temperature of from about 1060° C. toabout 1200° C. for a period of from about 1 to about 12 hours. Amongstthese particularly preferred embodiments of the invention, mostpreferred are those embodiments in which the metal substrate is heatedto a temperature of from about 1120° C. to about 1180° C. for a periodof 1 to about 4 hours.

The method employed for heating metal substrate 12 is not critccal andany conventional method can be used. For example, one convenient methodis to heat the metal substrate at an appropriate temperature for anappropriate period of time in a box furnace which allows some air topass in and out.

In the second step of the process of this invention a suspensioncomprising one or more organic solvents, one or more heat degradablepolymeric binders and a calcined mixture of finely divided glass/ceramiccomprising (on an oxide basis):

from about 8 to about 26% by weight of MgO;

from about 10 to about 49% by weight of Al₂ O₃ ; and

from about 42 to about 68% by weight of SiO₂, is applied to one or moresurfaces of a metal. The types of non-conductive materials employed inthe practice of this invention are as described above. Thenon-conductive materials are used in the form of finely dividedparticles. In the preferred embodiments of the invention, the materialsare in the form of finely divided spherical or substantially sphericalparticles having an average diameter of not more than about 10micrometers, and in the particularly preferred embodiments suchmaterials are in the form of finely divided spherical or substantiallyspherical particles having an average diameter of less than about 5micrometers. Amongst these particularly preferred embodiments mostpreferred are those embodiments in which non-conductive materials are inthe form of finely divided spherical or substantially sphericalparticles in which the average particle diameter is less than about 2micrometers.

"Thermally degradable polymeric organic binders" are one component ofthe suspension which is applied to the substrate in step two. As usedherein "thermally degradable polymeric organic binders" are naturallyoccurring or synthetic polymers which degrade when subjected to heat.Useful thermally degradable polymeric organic binders for use in thisinvention are also not critical and can also vary widely. Organicpolymer binders for use in the practice of this invention preferably arecapable of providing a stable colloidal suspension with the one or morenon-conducting materials and one or more organic solvents, and ispreferably thermally degradable when heated a temperature of at leastabout 300° C., and most preferably at a temperature from about 300° C.to about 600° C. to leave a substantially uniform sintered coating ofthe finely divided non-conductive material bonded to one or moresurfaces of the metal. Polymers which do not substantially completelythermally degrade or which leave decomposition products which interferewith the capacitive capability of the capacitor are not preferred foruse. In general, any naturally occurring or synthetic polymeric orelastomeric material can be used. Illustrative of such useful polymersare a, b unsaturated olefins such as polyvinyl alcohol, polyacrylates,polypropylene, polymethacrylates, polyvinyl chloride, polyethylene, andthe like; polyethers; polyesters such as polyethylene terephthalate,polybutylene terephthalate and the like; polyamides such as nylon-11,nylon-12, nylon-6, nylon-66 and the like; polysulfones; polyphenyleneoxides; cellulose based polymers, such as methyl cellulose ethers,ethylpropyl cellulose ethers and hydroxypropyl cellulose ethers; and thelike.

Another component of the suspension applied to the substrate in step twois one or more organic solvents. Organic solvents used to form thesuspensions are not critical, and can vary widely. The only requirementis that the solvents are capable of dissolving the thermally degradablepolymeric organic binders and capable of dispersing the one or morefinely divided non-conductive materials so as to form a colloidalsuspension or dispersion. In the preferred embodiments of the invention,organic solvents are those which volatilize when heated to a temperatureof from about 50° C. to about 250° C. at atmospheric pressure, and inthe particularly preferred embodiments organic solvents are those whichvolatilize when heated to a temperature of from about 75° C. to about150° C. at atmospheric pressure. Amongst these particularly preferredembodiments most preferred are those organic solvents which volatilizewhen heated to a temperature of from about 90° C. to about 120° C. atatmospheric pressure. Illustrative of useful solvents are alcohols,esters, ketones, aldehydes, hydrocarbons and like organic solvents.

The last component of the suspension applied to the substrate in steptwo is a mixture of calcined glass/ceramics. The calcined glass/ceramicmixture is formed by forming a mixture of finely divided Al₂ O₃, SiO₂,and MgO in the above- referenced proportions, together with one or more"effective fluxing and sintering agents" and optionally one or more"effective nucleating agents". As used herein, "effective fluxing andsintering agents" are fluxing and sintering agents which are effectiveto increase the adhesion of the ceramic glass to the metal substrate toany extent. These agents are precursor materials for the alkaline earthmetal and/or alkali metal oxides, or boron oxide contained in thecoating. Illustrative of effective fluxing and sintering agents arealkali metal salts such as LiBO₂, Li₂ O, B₂ O₃, Li₂ B₄ O₇, Na₂), NaBO₂,Li₄ B₆ O₁₁, K₂ O, CaO, LiF, NaF, LiCl NaCl, Na₂ SiF₆, Na₂ B₄ O₇,4CaO.5B₂ O₃.9H₂ O, LiAlSi₃ O₈, Li₂ Si₂ O₅, BaB₂ O₄, and the like.Preferred effective fluxing and sintering agents are alkali metalborates, meta-borates and like borates such as LiBO₂, Li₂ B₄ O₇, Li₂ Si₂O₅, LiF, BaB₂ O₄, 4CaO5B₂ O₃ 9H₂ O. Particularly preferred effectivefluxing and sintering agents are lithium borates and metaborates.Amongst these particularly preferred effective fluxing and sinteringagents are LiBO₂, Li₂ B₄ O₇, and LiF.

The amount of fluxing and sintering agents can vary widely. The amountof fluxing and sintering agents will depend on the amount of alkalineearth and/or alkali metal oxides, and boron oxides desired in theglass/ceramic composition. In general, the amount of such agents is fromabout 1 to about 15% based on the total weight of Al₂ O₃, MgO and SiO₂in the composition. In the preferred embodiments of the invention, theamount of fluxing and sinternng agents is from about 5 to about 12weight percent based on the total weight of Al₂ O₃, MgO and SiO₂ in thecomposition, and in the particularly preferred embodiments of theinvention the amount of fluxing and sintering agents is from about 7 toabout 9 on the aforementioned basis.

As used herein, "effective nucleating agents" are nucleating agentswhich promote the crystallization of residual glass phases. Illustrativeof such nucleating agents are metal oxides such as TiO₂, ZrO₂, Y₂ O₃,Ta₂ O₅ and La₂ O₃. Preferred nucleating agents are TiO₂ and ZrO₂, and aparticularly preferred nucleating agent is TiO₂.

The effective nucleating agents are optional when used, the amount mayvary widely. In general, the amount of such agents can vary from about 0to about 11% by weight based on the total weight of Al₂ O₃, MgO and SiO₂in the composition. In the preferred embodiments of the invention, theamount of effective nucleating agents is from 2 to about 8 weightpercent based on the total weight of MgO, Al₂ O₃ and SiO₂ in thecomposition, and in the particularly preferred embodiments of theinvention. The amount of effective nucleating agents is from about 4 to6 weight percent based on aforementioned basis.

Preparation of the calcine glass/ceramic mixture involves well knownprocedures. The raw materials are weighed and combined according to thedesired proportions. The raw materials can be added as pure oxides, oralternatively, in equivalent forms containing volatile species which areeliminated during subsequent heating. For example, magnesium carbonatecan be used in place of magnesium oxide, or boric acid can be used inplace of boron oxide. Similarly, the lithium oxide and boron oxide canbe conveniently added in a precombined form such as LiBO₂ or Li₂ B₄ O₇.The raw materials are thoroughly mixed, typically by wet milling in aball mill with added solvent, such as isopropanol in an amountsufficient to form a slurry of the desired proportion of ingredients.

After filtering and drying to form a powder, the materials are calcinedin order to produce the desired glass/ceramic composition. Calcinationmay be carried out using conventional procedures. For example, thisprocess may be carried out in a box furnace wtth a slow heat up to allowfor removal of any residual volatiles. Peak temperatures may vary,typically being in the range from about 1100° C. to about 1300° C., andtime at peak temperature may also vary, typically ranging from about 2to about 20 hours. The resultant product is then pulverized and ballmilled in an appropriate solvent such as isopropanol using standardceramic processing equipment to produce particles nominally less thanabout 10 microns in diameter. This slurry is then filtered and dried toproduce the fine powder which will subsequently be applied to a metalcore together with one or more binders and organic solvents.

The amounts of the various ingredients in the suspension employed in thefirst step of the process of this invention can vary widely. Very dilutesuspensions can be made for electrophoretic deposition and moreconcentrated suspensions for spraying, roller coating and the like. Thepreferred method in accordance with this invention is to make a highviscosity paste which is deposited in a screen printing process. Thepaste can be made with a variety of materials typical in the thick filmindustry. An illustrative formulation is a mixture of 65 weight percentceramic powder combined with an organic vehicle containing ethylcellulose dissolved in a high molecular weight alcohol. In general, thelower the concentration of suspended non-conductive materials in thesuspension the more often the suspension must be applied to the metal toprovide a given thickness of such bonded/sintered non-conductivematerial in the final glass-ceramic coated substrate; and conversely,the higher the concentration of suspended non-conductive materials inthe suspension, the less often the suspension must be applied to themetal to provide a given thickness of bonded/sintered non-conuuctivematerial in the glass-ceramic coated substrate. In general, theconcentration of organic solvents in the suspension will vary from about5 to about 50 weight percent, the concentration of the non-conductivematerials in the suspension will vary from about 40 to about 85 weightpercent, and the concentration of polymeric binders in the suspensionwill vary from about 1 to about 15 weight percent based on the totalweight of the suspension. In the preferred embodiments of the invention,the concentration of organic solvents in the suspension will vary fromabout 10 to about 45 weight percent, the concentration of non-conductivematerials in the suspension will vary from about 45 to about 80 weightpercent, and the concentration of polymeric binders in the suspensionwill vary from about 1 to about 10 weight percent. In the particularlypreferred embodiments, the concentration of organic solvents in thesuspension is from about 20 to about 40 weight percent, to concentrationof non-conductive materials in the suspension is from about 50 to about75 weight percent and the concentration of polymeric binders in thesuspension is from about 1 to about 5 weight percent. All weightpercents are based on the total weight of the suspension.

The suspension is applied to one or more surfaces of the metal in apredetermined pattern. The suspension can be applied over all of thesurfaces or over a portion thereof. Any suitable technique useful forapplying a suspension to the surface of a solid material can be used.Illustrative of useful techniques are screen printing, pad printing,dipping spraying and the like. Such techniques of applying suspensionsto a substrate are well known in the art and will not be describedherein in great detail. Application by use of screen printing ispreferred in accordance with this invention because of the ease withwhich patterns can be generated which allow open areas for mountingholes, ground plane interconnects, and electrically isolated vias fromone side to the other. Typically, the pattern is printed on both sidesof the metal in order to maintain uniform stress and eliminate bowing.

The suspension can be applied in a single application or multipleapplications can be made depending on the desired thickness of the layerof non-conductive material in the finished product. In the preferredembodiments there are from 1 to about 6 printed layers depending on thedesired thickness and in the most preferred embodiments of the inventiononly about 2 to about 3 layers. In some instances it is preferred thatthe final top layer ceramic composition differ from the underlying layeror layers. In this way one can take advantage of those ceramiccompositions with superior adhesive properties for direct contact to themetal core and those compositions with superior electrical propertiesfor contact with the overlying circuitry.

The amount of the suspension applied to the metal at any particularsitus will vary widely depending on the desired thickness of thenon-conductive material in the final glass-ceramic coated substrate. Theamount of the suspension applied to the metal is sufficient, usually, toprovide a layer of sintered non-conductive material bonded to the metalof a thickness of at least about 20 microns. In the preferredembodiments of the invention, the amount applied is sufficient toprovide a layer of non-conductive material having a thickness of fromabout 25 microns to about 85 microns, and in the particularly preferredembodiments, the amount applied is sufficient to provide a layer ofnon-conductive material having a thickness of from about 30 microns toabout 70 microns. Amongst these particularly preferred embodiments ofthis invention, most preferred are those embodiments in which the amountof suspension applied to the metal is sufficient to provide a layer ofsintered non-conductive material bonded to the metal having a thicknessof from about 40 microns to about 60 microns.

In the third step of the process of this invention, the metal to whichthe suspension has been applied in the desired predetermined pattern andin the desired amount heated at a temperature and for a time sufficientto remove substantially all of the organic solvent from the appliedsuspension and to sinter the non-conductive material, and to bond thesintered material to the metal substrate or substrates as the case maybe and to crystallize the residual glassy phase of the non-conductivematerial. The firing procedure is important in that it determines thedegree of adhesion and structure of the ceramic coating. The article ispreferably placed in a room temperature furnace which is subsequentlyprogrammed for a given temperature and time profile. In the preferredembodiments of the invention, the heating step is divided into twoportions and has at least two plateaus. In these preferred embodiments,the metal substrate to which the suspension has been applied is firstheated to a temperature sufficient to volatilize the solvents from thesuspension, preferably in less than about one hour without disturbingthe integrity of the remaining composition to form a coating of acomposition containing essentially no solvent and which comprises thefinely divided non-conductive material and the binders coated on thesurface of the substrate in the predetermined pattern. The purpose ofthis step is to ensure complete volatilization of the polymer binder sothat preferably substantially no carbon containing residual is present.The heating step can be carried out in an air atmosphere, or in anatmosphere of non-oxidizing gas. Obviously, this heating temperature canvary widely depending on the volatilization temperature of theparticular solvent or solvents employed. Usually, however, the heatingstep is carried out at a temperature equal to or less than about 200° C.for a period equal to or less than about 4 hours. In the preferredembodiments using preferred solvents, this heating step is carried outat a temperature of from about 50° C. to about 200° C. at atmosphericpressure for a period equal to or less than about 2 hours, and in theparticularly preferred embodiments of the invention using particularlypreferred solvents at a temperature of from about 75° C. to about 150°C. at atmospheric pressure for a period equal to or less than about 1hour. In the most preferred embodiments of the invention employing mostpreferred solvents, the first part of the heating step is carried out ata temperature of from about 90° C. to about 120° C. at atmosphericpressure for a period equal to or less than about 0.5 hour.

In the second step of the split heating procedure the metal and coatedcomposition from which the solvents have been substantially removed areheated at a temperature and for a time sufficient to degradesubstantially all of the polymer organic binders in the composition andsinter the finely divided non-conductive material and bond same to oneor more surfaces of the metal as the case may be, to produce asubstantially uniform coating of finely divided non-conductive materialon one or more surfaces of the metal. The heating temperature employedin the second part of the split heating step can vary widely and willdepend on the particular polymer binders, non-conductive materials andmetals employed and the temperature employed in the substrate heatingstep.

Polymer degrading and sintering can be carried out in a single step orin multiple steps. Preferably, polymer degrading and sintering arecarried in two steps. In the first step the polymer is degraded. In thisstp the coated substrate from which solvent has been removed is heatedat a temperature and for a time sufficient to degrade the polymer.Usually, this heating step is carried out at a temperature equal to orgreater than about 200° C. for a period equal to or less than about 4hours. In the preferred embodiments using preferred polymers, thepolymer degrading step is carried out at a temperature of from about200° C. to about 800° C. for a period equal to less or less than about 2hours and in the particularly preferred embodiments using particularlypreferred polymers is carried out at a temperature of from about 300° toabout 700° C. for a period equal to or less than about 1 hour. In themost preferred embodiments of the invention employing most preferredpolymers, the polymer degrading heating step is carried out at atemperature of from about 400° C. to about 600° C. for a period equal toor less than about 0.5 hour.

After the polymer has been degraded to the desired extent, the coatedsubstrate is then heated at a temperature and for a time sufficient tosinter the finely-divided non-conductive material and bond same to thesurface of the metal substrate. In general, the coated substrate isheated at a temperature below the melting point of the metal of thesubstrate for a period equal to or less than about 2 hours to sinter andbond the non-conductive material to the substrate. In the preferredembodiments of the invention, the coated substrate is heated at atemperature from about 1000° C. to about 1220° C. for a period equal toor less than 1 hour, and in the particularly preferred embodiments ofthe invention the coated substrate is heated at a temperature of fromabout 1060° C. to about 1200° C. for a period equal to or less than 0.5hour. Amongst these particularly preferred embodiments, most preferredare those embodiments in which the coated substrate is heated at atemperature of from about 1120° C. to about 1180° C. for a period equalto or less than 10 minutes. Subsequent cool down is not critical and thenatural cooling rate of the unpowered furnace is generally adequate.

During the solvent removal, sintering and polymer degrading stepsdensification and some vitrification of the glass/ceramic may occur.This residual glass structure is not desirable and can be substantiallyeliminated by a final heat treatment. In this heat treatment step, thecoated metal substrate is heated to a temperature and for a timesufficient to crystallize the residual glassy phase to the desiredextent. Heating temperatures and times may vary widely depending on thenature of the components of the coating. This procedure can be carriedout in a single step or in multiple steps. In the preferred embodimentof the invention, the procedure is carried out in two steps. In thefirst step, the coated metal substrate is usually heated to atemperature of from about 600° C. to about 900° C. for a period equal toor less than about 1 hour. In the preferred embodiments of theinvention, the substrate is heated to a temperature from about 650° C.to about 850° C. for a period equal to or less than about 0.5 hour, andin the most preferred embodiments of the invention, the coated substrateis heated for a period equal to or less than about 15 minutes at atemperature of from about 700° C. to about 800° C. In the second step ofthis split heating step, the coated substrate is heated at a temperatureof from about 800° C. to about 1100° C. over a period of from about 1 toabout 6 hours. In the preferred embodiments of the invention, theheating temperature is from about 800° C. to about 1050° C. and theheating period is from about 1 to about 6; and in the particularlypreferred embodiments of the invention, heating temperatures are fromabout 850° C. to about 1000° C. and heating times are from about 1 toabout 4 hours. Amongst these particularly preferred embodiments of thisinvention, most preferred are those embodiments in which heatingtemperatures of from about 900° C. to about 950° C. and heating times offrom about 1 to about 3 hours are employed.

The substrate so formed can be used to form electronic substrates. Forexample, suspension of a finely divided metal, as for example, the metalused as the solid metal substrate, such as copper, silver, goldaluminum, palladium, platinum and the like and alloys thereof, andcontaining one or more organic solvents having one or more polymericbinders dissolved therein is applied to surface of the sintered andbonded non-conductive material. The combination is thereafter heated tovolatilize substantially all of the solvents from the suspension and todegrade substantially all of the binders thereby sintering the finelydivided metal and bonding said sintered metal to the surface of thenon-conductive material. The suspension can be applied to one or moresurfaces of sintered and bonded non-conductive material in apredetermined pattern. The suspension can be applied over all of thesurfaces or a portion thereof using the same techniques employed in thesuspension application step 1. The suspension can be applied in a singleapplication or multiple applications can be made depending on thedesired thickness of the layer of metal in the finished electronicdevice. The components and the relative amounts of the components of thesuspension are as used in the suspension of step 1.

In an alternative embodiment of the process of this invention, thecombination of the metal and coated composition are used directly in thethird step of the process. In this procedure, the thermal degradation ofthe binders, and the sintering and heat bonding of both the sinterednon-conductive layer and the sintered metal layer are accomplished in asingle high temperature second part of the split heating step.

The solvent volatilization, polymer degradation, and sinteringprocedures used in this step are essentially the same as used in steptwo of the process of this invention in heating the suspension of thenon-conductive material to remove the solvents, degrading the polymerkinds, and sintering and bonding the resulting composition to thesurface of the solid conductive material. As in the case of the earlierheating step 2, the heating step of this procedure is preferably carriedout in two stages. In the first stage of the heating procedure, theapplied suspension is heated to a temperature and for a time which issufficient to volatilize the one or more solvents from the appliedsuspension. In the second stage of the heating procedure, the substratecoated with the dried suspension is heated at a temperature and for atime which is sufficient to sinter the finely divided metal, and bondthe sintered metal to the surface of the non-conductive layer. In thecase of the alternative embodiment of the process of this invention, thecoated substrate is also heated to a temperature and for a timesufficient to sinter the finely divided non-conductive material and bondthe material to the surface of the metal substrate. Usually in the caseof the alternative embodiment, the coating of the finely divided metaland the coating of the non-conductive material are sintered and bondedusing substantially the same heating conditions.

The thickness of the conductive layer is not critical and can varywidely. Usually, the layer has a thickness of from about 0.005 to about0.075 mm. In the preferred embodiments of the invention, the conductivelayer has a thickness of from about 0.01 to about 0.06 mm, and in theparticularly preferred embodiments of the invention has a thickness offrom about 0.015 to about 0.05 mm. Amongst these particularly preferredembodiments most preferred are those embodiments in which the conductivelayer has a thickness of from about 0.02 to about 0.03 mm.

The process of this invention can be used to manufacture electronicdevices of this invention. Such devices vary widely and include circuitboard, capacitors and the like. The process is preferred for use in themanufacture of circuit board.

The following specific examples are present to more particularlyillustrate the invention and are not intended to limit the scope of theinvention.

EXAMPLES 1 TO 18 General Procedure

A mixture of MgO, Al₂ O₃, and SiO₂, close to stoichiometric ratio of2:2:5 (cordierite) and about 4-10 wt % of Li₂ O . B₂ O₃ and/or Li₂ O. 2B₂ O₃ with 0-10 wt % TiO₂ or ZrO₂ is ball milled in isopropanol toinsure proper mixing and melt homogeniety. The slurry is then filtered,dried and calcined at 1100°-1300° C. for 4-12 hours. The solid productis then pulverized to form the compositions set forth in the followingTable I.

                  TABLE I                                                         ______________________________________                                                 Weight %                                                             Composition No.                                                                          MgO    Al.sub.2 O.sub.3                                                                      SiO.sub.2                                                                           Li.sub.2 O                                                                          B.sub.2 O.sub.3                                                                     TiO.sub.2                         ______________________________________                                        1          12.5   31.7    46.7  2.7   6.3   0.0                               2          12.4   31.4    46.2  3.0   7.0   0.0                               3          12.5   31.7    46.7  1.6   7.4   0.0                               4          12.4   31.4    46.2  1.8   8.2   0.0                               5          11.7   29.6    43.7  2.7   12.3  0.0                               6          12.5   32.0    47.0  2.6   5.9   0.0                               7           9.7   47.6    36.2  2.0   4.5   0.0                               8          22.7   20.0    45.4  2.4   5.5   4.0                               9          12.0   30.5    45.0  2.4   5.6   4.5                               10         12.3   31.2    45.9  1.8   4.2   4.6                               11         22.7   19.1    45.2  2.4   5.6   5.0                               12         11.2   28.3    41.7  2.5   5.8   10.5                              ______________________________________                                    

The pulverized product can be blended with a suitable organic vehicle tofrm a printable paste.

The substrate is a nickel alloy (214 Haynes Corporation). The surface ofthis alloy is first chemically etched for at least 30 minutes forinitial oxide removal, followed by heat treatment at a temperature atwhich the final product (coated substrate) is fired. This step providesan oxide layer which enhances the bonding between the metal and coating.This also prevents diffusion of metal into the coating, causingdiscoloration. After surface preparation, the metal is coated with theceramic paste via screen printing. The printed substrates are allowed tosettle at room temperature for 15-20 minutes followed by drying at 120°C. for 30-45 minutes. The dried samples are then fired in air at1140°-1300° C. for 1-15 minutes. The fired products can either bequenched at room temperature or allowed to cool slowly at the furnacecooling rate (ca. 40° C./minute). The final step in this process is heattreatment of the final products at 750°-1000° C. for 2-24 hours tocrystallize the residual glassy phase. The following Table II sets forththe processes and materials which produce good adhesion between theceramic and metal.

                  TABLE 2                                                         ______________________________________                                        Ex-  Metal Preparation           Firing Process                               am-             Heat Treatment                                                                            Ceramic                                                                              Peak                                       ple  Initial Oxide                                                                            in Air      Formu- Temp  Time                                 No.  Removal    (Temp/Time) lation °C.                                                                          (min)                                ______________________________________                                        1    FeCl.sub.3 /HCl                                                                           800° C./30 min                                                                    1      1150  20                                        Etch                                                                     2    FeCl.sub.3 /Hcl                                                                            800° C./30 min                                                                   1      1300  1                                         Etch                                                                     3    FeCl.sub.3 /HCl                                                                           800° C./30 min                                                                    2      1300  1                                         Etch                                                                     4    FeCl.sub.3 /HCl                                                                           800° C./30 min                                                                    3      1300  1                                         Etch                                                                     5    FeCl.sub.3 /HCl                                                                           800° C./30 min                                                                    4      1300  1                                         Etch                                                                     6    FeCl.sub.3 /HCl                                                                           800° C./30 min                                                                    5      1150  1                                         Etch                                                                     7    H.sub.2 SO.sub.4 /HCl                                                                    1200° C./30 min                                                                    7      1250  2                                         Etch                                                                     8    H.sub.2 SO.sub.4 /HCl                                                                    1200° C./30 min                                                                    7      1275  2                                         Etch                                                                     9    H.sub.2 SO.sub.4 /HCl                                                                    1200° C./30 min                                                                    8      1200  2                                         Etch                                                                     10   H.sub.2 SO.sub.4 /HCl                                                                    1200° C./30 min                                                                    8      1225  2                                         Etch                                                                     11   H.sub.2 SO.sub.4 /HCl                                                                    1200°  C./30 min                                                                   8      1250  2                                         Etch                                                                     12   H.sub.2 SO.sub.4 /HCl                                                                    1200° C./30 min                                                                    9      1200  2                                         Etch                                                                     13   H.sub.2 SO.sub.4 /HCl                                                                    1200° C./30 min                                                                    9      1225  2                                         Etch                                                                     14   H.sub.2 SO.sub.4 /HCl                                                                    1200° C./30 min                                                                    9      1250  2                                         Etch                                                                     15   H.sub.2 SO.sub.4 /HCl                                                                    1200° C./30 min                                                                    10     1225  2                                         Etch                                                                     16   H.sub.2 SO.sub.4 /HCl                                                                    1200° C./30 min                                                                    12     1200  2                                         Etch                                                                     17   H.sub.2 SO.sub.4 /HCl                                                                    1220° C./15 min                                                                    11     1175  2                                         Etch                                                                     18   Sand, 400 grit                                                                           1150° C./8 hrs                                                                     9      1160  10                                   ______________________________________                                    

EXAMPLE 19

Aggalloy was degreased and heat treated in air for 45 minutes at 1200°C. to build up an oxide layer. The metal was then printed with a pastecontaining dielectric composition 9, dried, fired at 1175° C. for threeminutes, and then heat treated at 900° C. for two hours. The resultantceramic had very good adhesion to the metal.

EXAMPLE 20

The identical dielectric and process procedure of Example 19 was usedwith Fecralloy as the supporting metal. Again, there was very goodadhesion of ceramic to metal.

EXAMPLE 21

Alloy 214 was etched in H₂ SO₄ /HCl for 30 minutes to remove the oxidesurface from as-received metal. It was then heat treated in air at 1200°C. for 30 minutes. Dielectric composition number 9 was printed with twolayers on each side of the metal. This was then followed by dielectriccomposition 6 printed in one layer on each side. The parts were fired at1180° C. for two minutes in a tube furnace followed by heat treatment at900° C. for 15 minutes in a box furnace.

Thick film Pd/Ag conductors were printed and fired (850° C./10 minutes)on these parts to determine electrical properties of the ceramicdielectric.

The parts were then subjected to repetitive thermal shock cycling as perMIL-STD-883C, Method 1011.5B between 125° C. and -55° C. liquid heatsinks. Results aee set forth in the following Table III.

                  TABLE III                                                       ______________________________________                                        Parameter        Initial    After 150 cycles                                  ______________________________________                                        Resistivity (ohm · cm)                                                                7.3 × 10.sup.13                                                                    6.9 × 10.sup.13                             Dielectric Constant (1 MHz)                                                                    6.5        6.7                                               Dissipation Factor (1 MHz)                                                                     0.0060     0.0062                                            ______________________________________                                    

Despite the large TCE difference (metal=13.3, ceramic=2.3), nocatastrophic failure was detected.

EXAMPLE 22

A series of tests were carried out to compare the substrateoof Example21 to alumina with regard to adhesion and conductivity of commerciallyavailable hybrid thick film materials applied to the surface of theceramic using a selection of conductive inks. Although applications forthick film on Al₂ O₃ are limited due to relatively complex (and costly)packaging schemes needed for mechanical fastening and to protect thefragile alumina from mechanical stress caused by shock, vibration andTCE mismatch, Al₂ O₃ was selected for evaluation because of its superiorelectronic properties.

Materials from several thick film past suppliers were printed on thesubstrate of Example 21 and compared to results on Al₂ O₃. The testsamples were screened on an MPM TF-100 Printer and fired in a BTUfurnace with a standard commercial 850° C. profile. The Aluminasubstrates were Coors 96% material.

Adhesion tests were performed on the four metallizations using theprocedure described in DuPont technical literature (80 mil squares)using an Instron model 1123. Conductivity of the four metallizations wasmeasured using a 500 Square serpentine pattern 0.010" wide.

Results of the study are shown in Table IV.

In the Table IV, the abbreviations have the following meanings:

(a) "ESL 9633B" is Ag/Pd paste manufactured and sold by ESL Corporation.

(b) "EMCA C3325" is Ag/Pd paste manufactured and sold by EMCACorporation.

(c) "DuPont 9770" is Ag/Pd paste manufactured and sold by DepontCompany.

(d) "Engelhard A3058" is Ag/Pd paste manufactured and sold by EnglehardtCorporation.

(e) "CCM" is the glass/ceramic coated substrate of Example 21.

                  TABLE IV                                                        ______________________________________                                        Properties of Thick Film Conductors                                           Fired at 850° C. for 10 minutes                                        Adhesion (lbs/0.080" square)                                                                               Conductivity                                                      After 24 hours                                                                            (Siemens/                                               Inital    at 150° C.                                                                         Square)                                          Conductor                                                                              COM     Al.sub.2 O.sub.3                                                                      COM   Al.sub.2 O.sub.3                                                                    COM   Al.sub.2 O.sub.3                   ______________________________________                                        ESL 9633B                                                                              5.4     5.9     4.0   4.3    11    21                                EMCA C3325                                                                             4.8     5.7     2.0   5.4    15    30                                DuPont 9770                                                                            6.1     5.9     3.0   3.0   100   200                                Engelhard                                                                              4.2     5.5     1.4   1.7   110   170                                A3058                                                                         ______________________________________                                    

In general, the range of conductivities available on the Example 21system overlaps quite well with alumina for the materials tested, andany reduced disadvantages attendent to conductivities are more thanoffset by the supperior mechanical properties of the Example 21substrate.

The results of Table IV show that for initial adhesion the adhesionproperties of the Comp 2 substrate are comparable to those of alumina.One substrate which was initially comparable to alumina by weakenedafter aging at 150° C. However, the final level achieved was stillbetter than the worst case on alumina with another material.

What is claimed is:
 1. A glass/ceramic coated substrate having a metalcore and having on at least a portion of the surface of (on an oxidebasis)(a) from about 8 to about 26% by weight of magnesium oxide (MgO);based on the total weight of the coating; (b) from about 10 to about 49%by weight of aluminum oxide (Al₂ O₃); based on the total weight of thecoating; (c) from about 42 to about 68% by weight of silicon oxide(SiO₂) based on the total weight of the coating and (d) from about 3 toabout 15% by weight, based on the total weight of MgO, Al₂ O₃ and SiO₂in the coating of one or more alkali metal or alkaline earth metaloxides, and a boron oxide.
 2. A glass/ceramic substrate according toclaim 1 wherein said coating comprises (on an oxide basis):(a) fromabout 9 to about 22% by weight of MgO; (b) from about 16 to 45% byweight of Al₂ O₃ ; and (c) from about 43 to about 63% by weight of SiO₂.3. A glass/ceramic substrate according to claim 2 wherein said coatingcomprises (on an oxide basis):(a) from about 10 to about 18% by weightof MgO; (b) from about 23 to about 40% by weight of Al₂ O₃ ; and (c)from about 44 to about 58% by weight of SiO₂.
 4. A glass/ceramicsubstrate according to claim 3 wherein said coating comprises (on anoxide basis):(a) from about 12 to about 14% by weight of MgO; (b) fromabout 30 to about 35% by weight of Al₂ O₃ ; and (c) from about 45 toabout 52% by weight of SiO₂.
 5. A glass/ceramic substrate according toclaim 1 wherein said coating comprises cordierete (2MgO-2Al₂ O₃ -5SiO₂).6. A glass/ceramic coated substrate according to claim 1 herein saidcoating comprises cordierete (2MgO-2Al₂ O₃ -5SiO₂) and sapphirine(4MgO-5Al₂ O₃ -2SiO₂).
 7. A glass/ceramic coated substrate according toclaim 1 which further comprises an alkali metal oxide and a boron oxide.8. A glass/ceramic coated substrate according to claim 1 which furthercomprises lithium oxide and boron oxide.
 9. A glass/ceramic coatedsubstrate according to claim 8 wherein said coating comprises cordierete(2MgO-2Al₂ O₃ -5SiO₂) and spodumene (Li₂ O-Al₂ O₃ -4SiO₂).
 10. Aglass/ceramic coated substrate according to claim 8 wherein said coatingcomprises cordierete (2MgO-2Al₂ O₃ -5SiO₂), spodumene (li₂ O-Al₂ O₃-4SiO₂), and sapphirine (4MgO-5Al₂ O₃ -2SiO₂).
 11. A glass/ceramicaccording to claim 1 which further comprises (on an oxide basis):(d)from about 5 to about 12% b weight based on the total weight of Al₂ O₃,MgO and SiO₂ in the coating, of one or more alkali metal and alkalineearth metal oxides and boron oxide.
 12. A glass/ceramic coated substrateaccording to claim 11 which further comprises (on an oxide basis):(d)from about 7 to about 9% by weight of one or more alkali metal and/oralkaline earth metal oxides, and boron oxides.
 13. A glass/ceramiccoated substrate according to claim 1 which further comprises (on anoxide basis)(e) up to about 11% by weight based on the total weight ofAl₂ O₃, SiO₃, SiO₂ and MgO of a transition or transition earth metaloxide which functions as a nucleating agent.
 14. A glass/ceramic coatedsubstrate according to claim 13 wherein the amount of said transition ortransition earth metal oxide is from about 2 to about 8% by weight. 15.A glass/ceramic coated substrate according to claim 14 wherein theamount of said transition or transition earth metal oxide is from about4 to about 6 weight percent.
 16. A glass/ceramic coated substrateaccording to claim 13 wherein said transition or transition earth metaloxide is selected from the group consisting of TiO₂, ZrO₂, Y₂ O₃ and La₂O₃.
 17. A glass/ceramic coated substrate according to claim 16 whereinsaid transition or transition earth metal oxide is selected from thegroup consisting of TiO₂ and ZrO₂.
 18. A glass/ceramic coated substrateaccording to claim 17 wherein said transition or transition earth metaloxide is TiO₂.
 19. A glass/ceramic coatd substrate according to claim 1said coating consisting essentially of (on an oxide basis and based onthe total weight of the coating):(a) from about 9 to about 23 weightpercent of magnesium oxide; (b) from about 19 to about 48 weight percentof aluminum oxide; (c) from about 36 to about 47 weight percent ofsilicon dioxide; (d) from about 1 to about 3 weight percent of lithiumoxide; (e) from about 4 to about 13 weight percent of a boron oxide; and(f) from about 0 to about 11 weight percent of titanium dioxide orzirconium oxide.
 20. A glass/ceramic coated substrate according to claim19 wherein said glass/ceramic has a thermal coefficient of expansion offrom about 50° to about 250° C. less than about 4 ppm/°C. and said metalcore has a thermal coefficient of expansion from about 50° to about 250°C. greater than about 12 ppm/°C.
 21. A glass/ceramic coated substrateaccorrding to claim 19 wherein said glass/ceramic has a dielectricconstant of less than about
 8. 22. A glass/ceramic coated substrateaccording to claim 19 in which the glass/ceramic consists essentially ofcordierite, 2MgO-2Al₂ O₃ -5SiO₂, and spodumene (Li₂ O-Al₂ O₃ -4SiO₂) 23.A glass/ceramic coated substrate according to claim 18 which includessapphirine (4MgO-5Al₂ O₃ -2SiO₂).
 24. A glass/ceramic coated substrateaccording to claim 23 which includes from about 2 to about 8% by weightof TiO₂.
 25. A glass/ceramic coated substrate according to claim 24wherein said glass/ceramic has a thermal coeffficient of expansion fromabout 50° C. to about 250° C. of from about 1.9 to about 2.5 ppm 1° C.26. A glass/ceramic coated substrate according to claim 1 wherein saidmetal core is composed of a metal which can be exposed to temperaturesin excess of 1100° C. in an oxidizing atmosphere without excessivespalling.
 27. A glass/ceramic coated substrate according to claim 26wherein said metal core is composed of an alloy of at least on metalselected from the group consisting of cobalt, nickel and iron, and oneor more other metals selected from the group consisting of aluminum andchromium.
 28. A glass/ceramic coated substrate according to claim 27 inwhich the core metal is comprised of a nickel based alloy containingchromium, aluminum, iron and yttrium.
 29. A glass/ceramic coatedsubstrate according to claim 28 in which the core metal is Haynes alloynumber 214 of nominal composition 77 weight % nickel, 16 weight %chromium, 4.5 weight % aluminum, 2.5 weight % iron, and a trace ofyttrium.
 30. A glass/ceramic coated substrate according to claim 27 inwhich the core metal is comprised of an iron based llloy containingchromium and aluminum.
 31. A glass/ceramic coated circuit board whichcomprises a metal substrate having coated on at least a portion of thesurface thereof a glass/ceramic coating and having defined on at least aportion of the surface of the coating an electrical circuit, saidcoating comprising (on an oxide basis;(a) from about 8 to about 26% byweight of MgO based on the total weight of the coating; (b) from about10 to about 49% by weight of Al₂ O₃ based on the total weight of thecoating; (c) from about 42 to about 68% by weight of SiO₂ and based onthe total weight of the coating; (d) from aobut 3 to about 15% byweight, based on the total weight of MgO, Al₂ O₃ and SiO₂ in the coatingof one or more alkali metal or alkaline earth metal oxides, and a baronoxide.
 32. A glass/ceramic coated circuit board according to claim 26said glass/ceramic having an oxide composition consisting essentiallyof:(a) from about 9 to about 23 weight percent of magnesium oxide; (b)from about 19 to about 48 weight percent of aluminum oxide; (c) fromabout 36 to about 47 weight percent of silicon dioxide; (d) from about 1to about 3 weight percent of lithium oxide; (e) from about 4 to about 13weight percent of boron oxide; (f) from about 0 to about 11 weightpercent of titanium dioxide or zirconium oxide; said coating havingdefined thereon an electrical circuit.
 33. The circuit board accordingto claim 32 wherein said glass/ceramic has a thermal coefficient ofexpansion from about 50° to about 250° C. less than about 4 ppm/°C. andsaid metal core has a thermal coefficient of expansion from about 50° toabout 250° C. greater than about 12 ppm/°C.
 34. The circuit boardaccording to claim 32 wherein said glass/ceramic has a dielectricconstant less than
 8. 35. The circuit board according to claim 32wherein said glass/ceramic consists essentially of cordierite,(2MgO-2Al₂ O₃ -5SiO₂), and spodumene (Li₂ O-Al₂ O₃ -4SiO₂).
 36. Thecircuit board according to claim 32 in which the core metal is comprisedof a nickel based alloy containing chromium, aluminum, iron, andyttrium.
 37. The circuit board according to claim 36 in which the coremetal is Haynes alloy number 214 of nominal composition 77 weight %nickel, 16 weight % chromium, 4.5 weight % aluminum, 2.5 weight % iron,and a trace of yttrium.
 38. The circuit board according to claim 32 inwhich the core metal is comprised of an iron based alloy containingchromium and aluminum.
 39. The circuit board according to claim 32 inwhich the glass/ceramic coating is deposited in two or more layers, saidlayers consisting of different compositions.
 40. The circuit boardaccording to claim 39 having a first layer with an oxide compositioncontaining about 12.0% magnesium oxide, about 30.5% aluminum oxide,about 45.0% silicon dioxide, about 2.4% lithium oxide, about 5.6% boronoxide, and about 4.5% titanium dioxide, and a second layer with an oxidecomposition containing about 12.5% magnesium oxide, about 32.0% aluminumoxide, about 47.0% silicon dioxide, about 2.6% lithium oxide, and about5.9% boron oxide.
 41. A process for manufacturing a glass ceramic coatedsubstrate having a metal core and having on at least a portion of thesurface of the metal core a coating of a glass ceramic, said processcomprising:(a) heating a metal substrate in the presence of oxygen at afirst temperature for a time sufficient to form an oxide layer on thesurfaces of said substrate; (b) applying to all or a portion of thesurfaces of said substrate in a pre-determined pattern a suspensioncomprising one or more organic solvents, one or more heat degradablepolymeric binders and a mixture of calcined finely divided comprising onan oxide basis:(i) from about 8 to about 26% by weight of MgO; (ii) fromabout 10 to about 49% by weight of Al₂ O₃ ; and (iii) from about 42 toabout 68% by weight of SiO₂ ; (c) heating the suspension/metal substratecombination of step (b) at a second temperature for a time sufficient toremove substantially all said solvents from the applied suspension toform a coated metal substrate; (d) heating said coated/metal substratecombinations of step (c) at a third temperature for a time sufficient todegrade substantially all of said binders in said applied suspension;(e) heating the coated metal substrate combintaion of step (d) for atime and a a fourth temperature sufficient to sinter said glass/ceramicto form a pre-determined pattern of sintered glass ceramic bonded tosaid surfaces of said metal substrate; and (f) heating treating saidsintered glass ceramic at a fifth temperature for a time sufficient torecrystallize any residual glass in said sintered glass/cermmic.
 42. Anelectronic substrate comprising:(a) a metal substrate; (b) a glassceramic coating comprising (on an oxide basis);(i) from about 8 to about26% by weight of MgO based on the total weight of the coating; (ii) fromabout 10 to about 49% by weight of Al₂ O₃ based on the total weight ofthe coating; (iii) from about 42 to about 68% by weight of SiO₂ based onthe total weight of the coating; (iv) from about 3 to about 15% byweight, based on the total weight of MgO, Al₂ O₃ and SiO₂ in the coatingof one or more alkali metal or alkaline earth metal oxides, and a boronoxide; and (c) a layer of a conductive material heat bonded and sinteredto all or a portion of the surfaces of said glass/ceramic coating in apre-determined pattern.
 43. An electronic substrate according to claim42 which further comprises a layer of resistive materials heated bond tosaid surfaces of said coating in a predetemined pattern forming aresistor between two or more circuit patterns.
 44. An electronicsubstrate according to claim 42 which further comprises one or morecapacitive elements said elements formed by overlapping circuitsandwhiching a layer of dielective material.
 45. An electronic substrateaccording to claim 42 wherein said substrate forms a circuit boardwherein said layer of a conductive material is heat bonded and sinteredto all or a portion of the surfaces of said glass/ceramic coating in apredetermined pattern defining an electrical circuit.
 46. An electronicsubstrate according to claim 45 wherein said conductive material is ametal.
 47. An electronic substrate according to claim 46 wherein saidmetal is selected from the group consisting of copper, silver, gold,platinum, palladium or alloys or other combinations thereof.
 48. Anelectronic substrate according to claim 42 wherein said substrate formsa capacitive device.